A laser machining apparatus for laser drilling a printed circuit board in a manufacturing process thereof needs a positioning control mechanism for irradiating a plurality of machining positions in a work piece with a laser beam sequentially. An optical scanner is often used to attain high machining throughput and high accuracy. The optical scanner is constituted by a rocking actuator and a servo controller. The rocking actuator includes a steerable mirror serving as a load element and fixed to a rotating shaft thereof. The servo controller controls the mirror so that the angle of the mirror can follow a command value.
The laser machining apparatus ordinarily serves as a numerical control (NC) system having a hierarchical control structure. The optical scanner belongs to the lowest class in the hierarchy. In a higher-class controller (hereinafter referred to as “supervisory controller”), based on CAM (Computer Aided Manufacturing) data of a printed circuit board, two-dimensional position coordinates of holes are described in an NC program in order of time when the holes should be machined. When machining is started, the supervisory controller transforms the hole position coordinates in the NC program sequentially, and transmits time-series angle command data to the optical scanner. To form a round hole in the printed circuit board, it is necessary to irradiate the printed circuit board with a laser beam after the steerable mirror has stood still at an angle commanded by the angle command data. Therefore, the transmission of the angle command data and the control of the irradiation with the laser beam are performed synchronously in the supervisory controller. The optical scanner operates to position the angle of the steerable mirror accurately correspondingly to the angle command data. Patent Document 1 discloses a technique for optimizing the laser drilling order in order to improve the throughput of a laser machining apparatus using the optical scanner.
Electromagnetic actuators are often used as rocking actuators. The electromagnetic actuators are categorized as moving-coil actuators or moving-magnet actuators. In a moving-coil actuator, a magnetic field is formed in an air gap between a permanent magnet of a stator and a yoke, and driving torque generated in the magnetic field by a moving coil according to Fleming's left hand rule is transmitted to a rotating shaft. In a moving-magnet actuator, which uses a coil as a stator and uses a permanent magnet as a movable element, driving torque generated by reaction of Fleming's left hand rule is received by the permanent magnet and transmitted to a rotating shaft. Patent Documents 2 and 3 disclose techniques about moving-magnet actuators. Patent Document 4 discloses a technique about a moving-coil actuator. In these actuators, rare-earth-based magnets such as neodymium iron boron magnets having a high residual flux density and a high coercive force are used as materials of permanent magnets in order to enhance the positioning responsiveness. Non-Patent Document 1 discloses a demagnetization curve, a temperature characteristic coefficient, etc. of a neodymium iron boron magnet.
Patent Document 5 discloses a technique about a motor having a rotor provided with a permanent magnet. In the motor, in order to reduce an eddy current loss which may be generated in the permanent magnet, the permanent magnet is axially or circumferentially split into a plurality, and insulators are provided among the split permanent magnets.
Patent Document 1: JP-A-2003-245843
Patent Document 2: Japanese Patent No. 3199813
Patent Document 3: JP-T-2003-522968
Patent Document 4: JP-A-2005-348462
Patent Document 5: JP-A-2005-354899
Non-Patent Document 1: Hi-Dong Chai, Electromechanical Motion Devices, Chap. 8, Prentice-Hall, 1998
The positioning responsiveness of the optical scanner, that is, the frequency of possible positioning motions per unit time is an essential factor in influencing the throughput of the laser machining apparatus. By use of the technique for optimizing the order of machining as disclosed in Patent Document 1, the frequency of short-stroke positioning motions can be increased. Thus, the throughput can be improved. In this case, the optical scanner performs the short-moving-distance positioning motions at a high speed. Accordingly, the optical scanner repeats the cycle of maximum acceleration, maximum deceleration and stop. Laser irradiation after the stop of the steerable mirror finishes in a short time. Accordingly, the time when the value of the coil current is 0 [A] is short, which causes a large copper loss. That is, Joule heat is generated by the current applied to the coil. When the heat is transmitted to the permanent magnet, the permanent magnet is demagnetized as described in Non-Patent Document 1. Particularly neodymium iron boron magnets have a large temperature coefficient of reversible demagnetization as compared with those of other magnet materials. The neodymium iron boron magnets fall 1.2 [%] in residual flux density and 6 [%] in coercive force every 10° C. of temperature rise. As a result, the gain of the feedback loop engaging in servo control declines so that the transient response such as an overshoot appears in the settling motion of the steerable mirror. Thus, the time required for positioning is prolonged.
An eddy current loss also causes a temperature rise of the permanent magnet. When the cycle of maximum acceleration, maximum deceleration and stop is repeated as described above, a current containing a high frequency component is applied to the coil. When the current is supplied in a pulse width modulation mode, frequency modulation components are contained in the current. The magnetic flux formed by the coil changes in accordance with these AC components. Accordingly, the eddy current appears in the permanent magnet opposed to the coil so that the temperature of the permanent magnet rises due to Joule heat generated by the eddy current. Cooling is therefore essential to obtain a high positioning responsiveness in the rocking actuator.
Patent Document 2 discloses a technique for transferring heat from a coil to a structure outside an actuator through a housing. Patent Document 3 discloses a technique for providing a heat sink and a cooling fan outside an actuator. A moving-magnet actuator has a yoke as a part constituting a stator. The yoke is provided outside a coil so that the yoke can transmit magnetic flux. Since the yoke is formed out of an iron-based material, the yoke is poor in heat conduction. However, Patent Documents 2 and 3 disclose no technique for solving the poor heat conduction of the yoke. Patent Document 4 discloses no technique for letting out the heat of the coil of the moving-magnet actuator.
Further, in the technique disclosed in Patent Document 5, the permanent magnet to be fixed to the rotor is split axially. Therefore, there is a fear that the torsional rigidity of the rotor may deteriorate. As described in BACKGROUND OF THE INVENTION, the angle of the movable element is servo-controlled in the rocking actuator. Accordingly, the servo control bandwidth is affected by a natural frequency of torsional vibration of the movable element. That is, when the torsional rigidity is low, the natural frequency also becomes low. Therefore in order to keep the feedback loop of the servo control stable, it is necessary to narrow the servo control bandwidth. When the servo control bandwidth is narrowed, there arises a problem that positioning responsiveness may be limited, or the positioning accuracy may deteriorate easily due to disturbance such as friction acting on the movable element.